Artist isn't breaking through (7)
I believe the answer is:
painter
'artist' is the definition.
(I know that painter is a type of artist)
'isn't breaking through' is the wordplay.
'isn't' becomes 'aint' (non-standard version of 'isn't').
'breaking' is an insertion indicator.
'through' becomes 'per' (I've seen this before).
'aint' placed within 'per' is 'PAINTER'.
(Other definitions for painter that I've seen before include "What Rolf Harris was allowed to do" , "Constable, say" , "keeps a boat by the bank" , "Cougar" , "Line attached to the bow of a boat when tying up" .)